The Japanese sika deep (Cervus nippon) proved to be a suitable animal model for the study of acute phases of in vivo erythrocyte sickling. Ophthalmologic studies can be conducted during or after 1 to 6 hours of effective in vivo sickling. Intravenous administration of 1.75 to 3.5% sodium bicarbonate solution at a rate of 500 to 1,000 ml per hour produced a transient state of alkalosis and in vivo erythrocyte sickling in the sika deer. The percentage of sickled erythrocytes increased as the blood pH increased. Concurrently, the packed cell volume decreased. Sickling was enhanced by 100% oxygen ventilation after endotracheal intubation and light anesthetization. After the induction of erythrocyte sickling, a sickling-reversal phenomenon occurred despite continued bicarbonate administration. During the course of this reversal, the percentage of sickled erythrocytes steadily decreased, the venous blood pH decreased, and the packed cell volume slowly increased. Because of the sickling-reversal phenomenon, chronic erythrocyte sickling was not achieved.